High-performance Jet Aircraft Research Articles

Methods for high-performance jet aircraft noise source imaging and sound field reconstruction

Source characterization and sound field reconstruction are vital to understanding noise radiation from turbulent, heated, supersonic jets produced by high-performance engines. This paper describes the application of two acoustic imaging methods to full-scale tactical aircraft noise analyses: statistically optimized near-field acoustical holography and hybrid beamforming. These methods constitute equivalent wave and equivalent source models, respectively. A review of these methods is given and their respective features are discussed.

Vibration-Induced Heating of Energetic Materials: A Review

AbstractThe transport of energetic materials—whether by truck over rough terrain, or attached to the undercarriage of a high-performance jet aircraft—carries a certain level of inherent risk as the repeatedly applied stresses from vibration may lead to heating, mechanical degradation, and potentially even the triggering of an ignition event. Increasing knowledge of the underlying physics which control ignition is allowing us to better understand, and thus reduce, the risk of a catastrophic event occurring. The Apollo and Space Shuttle programmes provided motivation for research into the topic in the 1960s and 1970s, and some recent studies have focussed on the grain-scale physics of ignition. However, much of the useful insight has arisen from work with other primary applications in mind. Therefore, this review aims to bring together literature from several fields, with the intention of better understanding vibration-induced heating (VIH) phenomena in energetic materials. Sensitivity, VIH in viscoelastic polymers and inert composites, and a technique known as vibrothermography which uses VIH to detect cracks, are all considered where relevant read-across can be found. Often being viscoelastic materials and composites with complex rheology, energetic materials subjected to vibrational loading tend to warm up, with potential for even greater temperature rises due to anisotropy-driven localised heating mechanisms. Binders soften as temperature rises, and the chance of damage increases, which may lead to runaway heating and thermal failure (if mechanical failure does not occur first).

Crew-Friendly Countermeasures Against Musculoskeletal Injuries in Aviation and Spaceflight.

Aviation and space medicine face many common musculoskeletal challenges that manifest in crew of rotary-wing aircraft (RWA), high-performance jet aircraft (HPJA), and spacecraft. Furthermore, many astronauts are former pilots of RWA or HPJA. Flight crew are exposed to recurrent musculoskeletal risk relating to the extreme environments in which they operate, including high-gravitational force equivalents (g-forces), altered gravitational vectors, vibratory loading, and interaction with equipment. Several countermeasures have been implemented or are currently under development to reduce the magnitude and frequency of these injuries. Cervical and lumbar spine, as well as extremity injuries, are common to aviators and astronauts, and occur in training and operational environments. Stress on the spinal column secondary to gravitational loading and unloading, ± vibration are implicated in the development of pain syndromes and intervertebral disk pathology. While necessary for operation in extreme environments, crew-support equipment can contribute to musculoskeletal strain or trauma. Crew-focused injury prevention measures such as stretching, exercise, and conditioning programs have demonstrated the potential to prevent pre-flight, in-flight, and post-flight injuries. Equipment countermeasures, especially those addressing helmet mass and center of gravity and spacesuit ergonomics, are also key in injury prevention. Furthermore, behavioral and training interventions are required to ensure that crew are prepared to safely operate when faced with these exposures. The common operational exposures and risk factors between RWA and HPJA pilots and astronauts lend themselves to collaborative studies to develop and improve countermeasures. Countermeasures require time and resources, and careful consideration is warranted to ensure that crew have access to equipment and expertise necessary to implement them. Further investigation is required to demonstrate long-term success of these interventions and inform flight surgeon decision-making about individualized treatment. Lessons learned from each population must be applied to the others to mitigate adverse effects on crew health and well-being and mission readiness.

Effect of a Recirculating Type Casing Treatment on a Highly Loaded Axial Compressor Rotor

The tip leakage flow over the blades of an axial compressor rotor adversely affects the axial rotor efficiency and can determine the onset of tip leakage stall. The performance of a new casing treatment concept in the shape of an axisymmetric recirculation channel is explored by steady Reynolds-Averaged Navier–Stokes (RANS) realizable k-ε modelling on the NASA Rotor 37 test case. The modelling exposed a number of attractive features. The casing treatment increased the stall margin at no penalty to the rotor isentropic efficiency over the rotor operating line. A recirculation in the casing channel self-activated and self-adjusted with the rotor loading to provide more passive flow control at higher rotor loading conditions. The nozzle-shaped recirculation channel outflow opposed the tip leakage jet, re-located the casing surface flow interface further downstream, and reduced the rotor blade tip incidence angle. This combination of features makes the new casing treatment particularly attractive for applications to high thrust-to-weight ratio engines, typical of high-performance jet aircraft.

Comparing two inverse array methods for source reconstructions of noise radiated from a high-performance jet aircraft

Because direct flow measurements of tactical aircraft jet engines are not currently possible, acoustic source characteristics are instead inferred from array processing. This paper compares two array processing methods using the same array data from a high-performance military aircraft. Hybrid beamforming (HBF) and multisource statistically optimized near-field acoustical holography (M-SONAH) have both been used previously for frequency-dependent jet noise source characterization, but are compared here for the same input data. A 71-element linear array of equally spaced microphones was placed approximately parallel to the shear layer covering a distance of 32 m. Complex pressures obtained from this array served as the input to both methods. Favorable agreement in terms of maximum source location, source shape, and source extent was seen between the two methods' respective results. While the methods continue to have their relative strengths and reasons for use, this favorable agreement indicates that an improved understanding of military jet noise sources has been achieved. [Work supported by an AFRL SBIR.]

Characterizing acoustic shocks in high-performance jet aircraft flyover noise.

Acoustic shocks have been previously documented in high-amplitude jet noise, including both the near and far fields of military jet aircraft. However, previous investigations into the nature and formation of shocks have historically concentrated on stationary, ground run-up measurements, and previous attempts to connect full-scale ground run-up and flyover measurements have omitted the effect of nonlinear propagation. This paper shows evidence for nonlinear propagation and the presence of acoustic shocks in acoustical measurements of F-35 flyover operations. Pressure waveforms, derivatives, and statistics indicate nonlinear propagation, and the resulting shock formation is significant at high engine powers. Variations due to microphone size, microphone height, and sampling rate are considered, and recommendations for future measurements are made. Metrics indicating nonlinear propagation are shown to be influenced by changes in sampling rate and microphone size, and exhibit less variation due to microphone height.

Measurement methods for high-performance jet aircraft noise inside a hardened aircraft shelter

High-performance military aircraft regularly operate inside hardened aircraft shelters (HAS). The F-35A aircraft must be certified as safe to operate inside a HAS before it can be deployed and used in such structures worldwide. Acoustic levels at maintainer locations allow for noise dose estimates and regulation of personnel mission support in HASs to mitigate risks of hearing damage. Acoustic levels impinging on the airframe are compared against engineering design limits in order to prevent a reduction in operational lifespan of the aircraft due to acoustic fatigue. The Air Force Research Laboratory, the Royal Netherlands Air Force, and the Dutch national laboratories NLR and TNO collaborated on a set of acoustic measurements for an F-35A operating inside a HAS at Leeuwarden airbase in the Netherlands. The methods, analysis, and qualitative findings of the acoustic measurements are presented here. [Work supported by RNLAF and by the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 01/24/2017; JSF17-035.]

Modeling shock formation and propagation in high-performance jet aircraft noise

Nonlinear propagation can play an important role in both time and frequency-domain features of far-field supersonic jet noise. Many aspects of nonlinear propagation, such as waveform steepening and greater-than-expected high-frequency spectral levels, have been previously predicted for select angles and engine conditions. This paper builds on previous successes and presents a comparison of nonlinear and linear predictions for the F-35B aircraft. Results are shown over a wide spatial and angular range and over varying engine power conditions, including showing evidence of nonlinear propagation in the forward direction at the highest engine conditions. In addition, specific features, such as individual shocks, are compared between numerically propagated and measured waveforms, highlighting the successes and deficiencies of current propagation models. Weather and multi-path interference effects are also addressed and corrected using an empirical model. [Work supported by USAFRL through ORISE. Distribution A: Approved for public release; distribution unlimited; Cleared 07/10/2017; JSF17-714.]

Evidence for -Gz Adaptation Observed with Wearable Biosensors During High Performance Jet Flight.

Few studies have evaluated physiological responses to high acceleration forces during actual flight and to our knowledge no normative data has been acquired by technologies such as wearable biosensors during high performance jet aircraft operations. In-flight physiological data from an FDA cleared portable triaxial accelerometer and bio-sensor were observed from five active duty F-18 pilots of the Naval Flight Demonstration Squadron (Blue Angels). Of the five pilots, three were formation pilots who flew lower G profiles and two were solo pilots who flew higher G profiles. Physiological parameters monitored were heart rate, respiratory rate, temperature, caloric expenditure, and duration of exposure to levels of acceleration. Evaluated were 25 practice demonstration flights; 9 flights were excluded secondary to incomplete or inaccurate physiological data. We observed no significant bradycardia during a total of 189 maneuvers which met inclusion criteria for push-pull events (PPE) or isolated -Gz exposures. Further analysis of 73 PPE revealed an overall significant rise in HR following the PPE, where mean heart rate was 106 (95% CI, 100:112) at the beginning of the push and 129 (95% CI, 123:135) following the pull. A majority of the flights monitored provided reliable physiological data. Initial data suggests, contrary to currently held aeromedical doctrine, maneuvers such as the "push-pull" do not evoke vasovagal based bradycardic responses in aerobatic pilots. Possible explanations for these findings are sympathetic nervous system activation through adaptation and/or sustained isometric resistance from control inputs, both of which are areas of future research for our team.Rice GM, Snider D, Moore JL, Lavan JT, Folga R, VanBrunt TB. Evidence for -Gz adaptation observed with wearable biosensors during high performance jet flight. Aerosp Med Hum Perform. 2016; 87(12):996-1003.

Comparison of nonlinear, geometric, and absorptive effects in high-amplitude jet noise propagation

In recent years, understanding of nonlinearity in noise from high-performance jet aircraft has increased, with successful modeling of nonlinear propagation in the far field. However, the importance and characteristics of nonlinearity in the near field are still debated. An ensemble-averaged, frequency-domain version of the Burgers equation can be inspected to directly compare the effects of nonlinearity on the sound pressure level with the effects of atmospheric absorption and geometric spreading on a decibel scale. This nonlinear effect is calculated using the quadspectrum of the pressure and the squared pressure waveforms. Results from applying this analysis to F-22A data at various positions in the near field reveal that in the near field the nonlinear effects are of the same order of magnitude as geometric spreading and that both of these effects are significantly greater than absorption in the area of maximum radiation. [Work supported by ONR and an ORISE fellowship through AFRL.]

Simulated sound in advanced acoustic model videos

The Advanced Acoustic Model (AAM) and other time-step aircraft noise simulation models developed by Wyle can generate video animations of the noise environment. The animations are valuable for understanding details of noise footprints and for community outreach. Using algorithms developed by NASA, audio simulation for jet aircraft noise has recently been added to the video capability. Input data for the simulations consist of AAM’s one-third octave band sound level time history output, plus flight path geometry and ground properties. Working at an audio sample rate of 44.1 kHz and a sample “hop” period of 0.0116 s, a random phase narrow band sample is shaped to match spectral amplitudes. Ground reflection and low frequency oscillation are added to the hops, which are merged into a WAV file. The WAV file is then mixed with an existing animation generated from the same AAM run. The process takes place in near-real time, based on a location that a user selects from a site map. The presentation includes demonstrations of the results for a simple level flyover and for the departure of a high performance jet aircraft from an airbase.

Do recent findings on jet noise answer aspects of the Schultz curve?

Recent research efforts on nonlinear propagation from high performance jet aircraft have revealed an interesting challenge to predicting community response. This challenge focuses on receiver perception of these unique acoustical signals, which contain acoustical shocks that appear to increase their relative loudness and/or noisiness. This current finding suggests a need for an improved description of a receiver perception of the loudness of these signals in order to improve the assessment of noise impacts from these aircraft. Looking backwards, an interesting question emerges: did the earlier low bypass jet engines on commercial and transport aircraft also include these acoustical shocks? If they did contain these features, then the perceptual differences observed between aircraft and other transportation noise sources may be partially explained.

Similarity spectra analysis of high-performance jet aircraft noise

Noise measured in the vicinity of an F-22A Raptor has been compared to similarity spectra found previously to represent mixing noise from large-scale and fine-scale turbulent structures in laboratory-scale jet plumes. Comparisons have been made for three engine conditions using ground-based sideline microphones, which covered a large angular aperture. Even though the nozzle geometry is complex and the jet is nonideally expanded, the similarity spectra do agree with large portions of the measured spectra. Toward the sideline, the fine-scale similarity spectrum is used, while the large-scale similarity spectrum provides a good fit to the area of maximum radiation. Combinations of the two similarity spectra are shown to match the data in between those regions. Surprisingly, a combination of the two is also shown to match the data at the farthest aft angle. However, at high frequencies the degree of congruity between the similarity and the measured spectra changes with engine condition and angle. At the higher engine conditions, there is a systematically shallower measured high-frequency slope, with the largest discrepancy occurring in the regions of maximum radiation.

Evolutionary design of a full-envelope full-authority flight control system for an unstable high-performance aircraft

The use of an evolutionary algorithm in the framework of H∞ control theory is being considered as a means for synthesizing controller gains that minimize a weighted combination of the infinite norm of the sensitivity function (for disturbance attenuation requirements) and complementary sensitivity function (for robust stability requirements) at the same time. The case study deals with a complete full-authority longitudinal control system for an unstable high-performance jet aircraft featuring (i) a stability and control augmentation system and (ii) autopilot functions (speed and altitude hold). Constraints on closed-loop response are enforced, that representing typical requirements on airplane handling qualities, that makes the control law synthesis process more demanding. Gain scheduling is required, in order to obtain satisfactory performance over the whole flight envelope, so that the synthesis is performed at different reference trim conditions, for several values of the dynamic pressure, used as the scheduling parameter. Nonetheless, the dynamic behaviour of the aircraft may exhibit significant variations when flying at different altitudes, even for the same value of the dynamic pressure, so that a trade-off is required between different feasible controllers synthesized at different altitudes for a given equivalent airspeed. A multi-objective search is thus considered for the determination of the best suited solution to be introduced in the scheduling of the control law. The obtained results are then tested on a longitudinal non-linear model of the aircraft.

Attenuation performance of double hearing protection devices.

High performance jet aircraft are capable of producing 130-150 dB of noise. Large populations work long hours in these high level noise environments and are at risk of hearing loss and related hearing disabilities. An array of hearing protection devices (HPD), both single hearing protection (SHP) and double hearing protection (DHP), are issued to protect aircrew and ground crew members from the nearly constant assault of hazardous noise. American National Standard Institute (ANSI) Methods for Measuring the Real-Ear Attenuation (REAT) of Hearing Protectors S12.6 were used to measure attenuation performance of nearly 20 SHP and DHP configurations. The term DHP can be slightly deceiving. DHP means two devices are worn in combination, not the sum of the attenuation from each device worn singly. The passive attenuation performance of inserts (earplugs) and helmets worn singly and in combination were reported using REAT octave band data, EPA Noise Reduction Ratings (NRR), and ANSI Methods of Estimating Effective A-Weighted Sound Pressure Levels when Hearing Protectors are Worn: the Noise Level Reduction Statistics for use with A-Weighting (NRSA), and the Noise Level Reduction Statistic, Graphical (NRSG).

Quantitative Fault Tolerant Control Design for a Leaking Hydraulic Actuator

This paper documents the design of a low-order, fixed-gain, controller that can maintain the positioning performance of an electrohydraulic actuator operating under variable load with a leaking piston seal. A set of linear time-invariant equivalent models of the faulty hydraulic actuator is first established, in the frequency domain, by Fourier transformation of acceptable actuator input-output responses. Then, a robust position control law is synthesized by quantitative feedback theory to meet the prescribed design tolerances on closed-loop stability and reference tracking. The designed fault tolerant controller uses only actuator position as feedback, yet it can accommodate nonlinearities in the hydraulic functions, maintain robustness against typical parametric uncertainties, and maintain the closed-loop performance despite a leakage fault that can bypass up to 40% of the rated servovalve flow across the actuator piston. To demonstrate the utility of the fault tolerant control approach in a realistic application, the experimental fault tolerant hydraulic system is operated as a flight surface actuator in the hardware-in-the-loop simulation of a high-performance jet aircraft.

Risk of Herniated Nucleus Pulposus Among U.S. Astronauts

Astronauts have complained of back pain occurring during spaceflight, presumably due to the elongation of the spine from the lack of gravity. Herniated nucleus pulposus (HNP) is known to occur in aviators exposed to high Gz and has been diagnosed in several astronauts in the immediate post-spaceflight period. It is unknown whether astronauts exposed to microgravity are at added risk for developing HNP in the post-spaceflight period due to possible in-flight intervertebral disc changes. For a preset study period, incidence rates of HNP were compared between the U.S. astronaut population and a matched control population not involved in spaceflight using the Longitudinal Study of Astronaut Health database. Using a Weibull survival model, time trends of the risk of HNP prior to and after spaceflight were compared within the astronaut group. HNP incidences in other populations that have previously been reported in the literature were also compared with results in this study. The incidence of HNP was 4.3 times higher in the U.S. astronaut population (N=321) compared to matched controls (N=983) not involved in spaceflight. For astronauts, there was relatively more HNP in the cervical region of the spine (18 of 44) than for controls (3 of 35); however, there was no clear increase of HNP incidence in those astronauts who were high performance jet aircraft pilots. There was evidence suggesting that the risk is increased immediately after spaceflight. Astronauts are at higher risk of incurring HNP, especially immediately following spaceflight.

Hardware-in-the-loop simulator for research on fault tolerant control of electrohydraulic actuators in a flight control application

Testing fault tolerant control and diagnosis systems for hydraulic actuators under operating conditions that closely resemble the intended application is an important part of control system verification. This paper describes a hardware-in-the-loop (HIL) simulation framework that enables an experimental hydraulic actuator to be exercised as a flight control actuator against the numerical simulation of a high-performance jet aircraft. The HIL simulator is centered around a state-of-the-art hydraulic test bench that can experimentally simulate many of the common component faults in a servovalve driven actuator circuit. A second experimental hydraulic ram is used to create a wide range of in-flight operating conditions by replicating the aerodynamic load disturbance that is transmitted through the control surface hinge during flight maneuvers. The goal of the HIL simulator is to develop and objectively test novel fault tolerant control and diagnostics algorithms for fluid power actuators and to investigate, for the first time, the complex interaction between faulty flight actuator hardware and the overall aircraft performance. An overview of the HIL simulation architecture, which is executed on four networked desktop computers, is given and the hardware implementation is described. Issues related to the design of the force control system around the load emulator actuator are also discussed. The results of several HIL case studies, including a human-in-the-loop simulation, are presented to demonstrate the functionality of the HIL simulation environment.

The Canberra high altitude mission platform

This presents the capabilities of two English Electric Aviation Canberra aircraft owned by High Altitude Mapping Missions, Inc. These aircraft are available for high altitude commercial, government, and scientific missions. The basic specifications of the Canberra are provided and compared to other high-performance jet aircraft falling into the same category. This describes the high altitude flight characteristics and mission capabilities of the Canberras. The aircraft are configured to conduct up to eight-hour flight operations with up to six hours at 50,000 feet altitude. Shorter missions can be conducted at higher altitudes. Potential missions include: IFSAR, magnetic field mapping, gravity field mapping, imaging (SAR, IR, & hyper-spectral), LIDAR, communications relay, dropsonde deployment, hybrid rocket launching, severe storm monitoring, and disaster monitoring. Canberra high altitude operations have minimal weather impact on missions, are free from flight constraints imposed by air traffic control, can do large area coverage, and long mission times.

Measured thrust losses associated with secondary air injection through nozzle walls

The experimental program discussed herein addressed the aerodynamic effects of auxiliary air on the thrust of a scaled two-dimensional converging/diverging nozzle. The tests were intended as a means of validating certain analytical and computational fluid dynamics (CFD) models for the thrust loss associated with cooling air injection. The ratios of injected flow rate to mainstream flow rate employed are typical of those used to cool the internal surfaces of high-performance jet aircraft nozzles. The primary purpose of the investigation was to establish a data base of nozzle performance sensitivity to various secondary air parameters. Divergent section pressure distributions obtained in the presence of across the flaps indicate that the injected flow constricts the primary flow thus reducing its effective expansion ratio. The results obtained also show a definite dependence of performance on the angle at which the secondary air is injected. The thrust coefficient changes resulting from injection, relative to the no injection situation, were reasonably predicted by a simple one-dimensional formulation corrected for certain two-dimensional flow effects. This formulation was observed to apply for injected flow rates up to about 6% of the primary flow rate. Nomenclature A = area Cd = discharge coefficient Q = coefficient used to modify one-dimensional stream thrust